1. Field of the Invention
The present invention relates to a process for the production of a powdery magnetic material for a magnetic recording medium. More particularly, the present invention relates to a process for the production of a novel powdery metal magnetic material.
2. Description of the Prior Art
Hitherto, .gamma.-Fe.sub.2 O.sub.3, Co containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co containing Fe.sub.3 O.sub.4, CrO.sub.2, and the like have been used as a ferromagnetic powder for use in a magnetic recording medium. These ferromagnetic powders, however, have not been suitable for magnetic recording of a signal of a short recording wave length, e.g., 10 microns or less. That is, the magnetic properties such as coercive force (Hc), maximum residual magnetic flux density (Br), and the like are insufficient for use in the so-called high density recording. Recently, ferromagnetic powders having properties suitable for high density recording have been extensively investigated. One of these ferromagnetic powders is a ferromagnetic metal powder. The powder is made of a metal or a metal alloy. Metals such as iron, cobalt, and nickel are mainly used, and, if desired, chromium, manganese, rare earth elements, zinc, and the like are added.
The following methods are known for producing a ferromagnetic substance.
(1) A method comprising decomposing an organic salt of a ferromagnetic metal by heating followed by a reduction thereof in a reducing atmosphere, as described in, for example, Japanese Patent Publication Nos. 11412/1961, 22230/1961, 8027/1965, 14818/1966, 22394/1968, 38417/1972, etc., and The Record of Electrical and Communication Engineering Conversazione Tohoku University, Vol. 33, No. 2, page 57 (1964).
(2) A method comprising reducing a needle-like oxyhydroxide compound or those compounds containing other metals in addition to the needle-like oxyhydroxide compound, or needle-like iron oxide produced from the oxyhydroxide compound, as described in, for example, Japanese Patent Publication Nos. 3862/1960, 20939/1964, 39477/1972, etc., German Patent Laid Open No. 2,130,921, British Pat. No. 1,192,167, U.S. Pat. No. 3,681,018, etc.
(3) A method comprising evaporating a ferromagnetic metal in an inert gas, as described in, for example, Japanese Patent Publication No. 27718/1972, and Ohyo Butsuri (Applied Physics), Vol. 40, No. 1, page 110 (1971).
(4) A method comprising decomposing a metal carbonyl compound, as described in, for example, U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007, 3,228,882, etc.
(5) A method comprising electro-depositing a ferromagnetic metal using a mercury cathode followed by the separation of the metal from the mercury, as described in, for example, Japanese Patent Publication No. 15525/1964, 8123/1965, etc., and U.S. Pat. No. 3,156,650.
(6) A method comprising reducing a salt of a ferromagnetic metal in a solution thereof, as described in, for example, Japanese Patent Publication Nos. 20520/1963, 26555/1963, 20116/1968, 41718/1972, etc., U.S. Pat. Nos. 3,206,338, 3,494,760, 3,567,525, 3,535,104, 3,607,218, 3,663,318, 3,669,643, 3,672,867, 3,756,866, German Patent Laid Open No. 2,132,430, 2,326,258, 2,326,261, etc.
The present invention is concerned with the Method (6) wherein a ferromagnetic metal salt is reduced in a solution thereof, and particularly, a borohydride compound or a derivative thereof is used as a reducing agent.
Method (6) of reducing a ferromagnetic metal salt in a solution thereof using a borohydride compound or a derivative thereof, has the following defects. That is, in general, in order to provide magnetic anisotropy, the reaction is effected in a magnetic field to thereby cause the particles to form a chain, and thus shape anisotropy is obtained. However, when the particle chains are mixed with and dispersed in a binder, they are broken, resulting in a reduction in the shape anisotropy. Thus, a tape produced using particle chains has tended to be inferior in the orientation in magnetic field, and poor in squareness ratio, Br/Bs, (where Br is the residual magnetic flux density and Bs is the saturated magnetic flux density).
Furthermore, a powder produced by the above described method is not humidity-resistant, particularly in the case where Fe is present, and thus the powder is gradually oxidized even though the powder is stored in an aqueous reaction solution or in air at an ordinary temperature, and, in the extreme cases, the magnetic properties are lost.
Moreover, the surface activity of the particles produced by the above described method is high, i.e., the particles are highly reactive, and thus they are industrially disadvantageous from the standpoint of process control.
Hitherto, various methods have been proposed in order to remove the above described drawbacks. For instance, Japanese Patent Publication No. 20520/1963 describes Fe-B based magnetic recording materials containing Co, Ni, Mn, and Cr prepared by a process which comprises applying either of the solutions of a hydrophilic high molecular weight material containing a salt of a ferromagnetic metal or a hydrophilic high molecular weight material containing a borohydride to a support to form a layer of the hydrophilic high molecular weight material on the support, applying the other solution onto the layer to precipitate the ferromagnetic metal particles in a dispersed state in the hydrophilic high molecular weight material. Japanese Patent Publication No. 7820/1972 describes Fe-B based materials containing Co, Ni, Mn, Cu, and Ag, prepared by a process which comprises reducing a solution of at least one ferromagnetic metal salt with a borohydride with the improvement comprising adding a surface active agent to the solution. Further Japanese Patent Publication No. 20116/1968 describes a process for preparing an oxidation-resistant wet-process magnetic metal powder which comprises adding sodium or potassium borohydride to a mixture (A) an aqueous solution of a salt of iron, cobalt, nickel, etc.) and (B) a dispersion of a higher aliphtic acid which is liquid at ambient temperature in the presence or absence of an additive which is capable of liberating a higher aliphatic acid upon hydrolysis of the additive thereby reducing the magnetic metal salt to precipitate the metal particles and at the same time saponify the surface of the metal particles with the higher aliphatic acid. However, it has been difficult to remove the above described drawbacks using these methods.